The invention relates to an ion irradiation system which utilizes an ion microprobe and, more particularly, to an ion irradiation system and method which permit highly accurate irradiation of a target with a single ion or predetermined number of ions placed under control.
With the progress of information processing techniques, there has been a growing demand for various satellites equipped with a variety of functions, and to meet the demand, semiconductor devices fabricated as closely-packed or high-density integrated circuits have come to be loaded on various satellites. In outer space a large number of radiant rays or cosmic rays are present, to which are exposed the semiconductor devices loaded on the various satellites, and their deterioration and temporary malfunction (such as a single event effect) by the radiant rays in outer space pose serious problems. The deterioration and temporary malfunction of the semiconductor devices by the radiant rays are likely to be caused by radioactive decay on the earth as well as in outer space, and much study is now being given to this problem in the development of closely-integrated semiconductor devices such as a DRAM (Dynamic Random Access Memory).
The integrated circuits that are loaded on an artificial satellite are subjected to very severe endurance tests prior to launching, because the circuits, once broken down, cannot be replaced after launching. In conventional irradiation tests for checking the durability of integrated circuits against high-energy ions, the integrated circuits are randomly or indiscriminately irradiated with ions over a wide area and statistics on the probability of occurrence of malfunction is compiled to thereby examine the durability of the integrated circuits against the ion irradiation. With this method, however, it is impossible to specify regions or sites where the integrated circuits are likely to malfunction, and hence the prior art lacks a decisive factor in the development of highly durable integrated circuits.
The phenomenon that is caused by the incidence of a single ion is commonly referred to as a single event. The single event gives rise to malfunction of a device, and hence constitutes an obstacle to the miniaturization or microfabrication of devices. To overcome this obstacle, it is of importance to clarify the mechanism of the single event and take preventive measures accordingly. A soft error and a latch-up are known as the device malfunction by the single event.
(i) Soft error
A dynamic random access memory (hereinafter referred to as a DRAM) stores information by storing charges in capacitors. The DRAM package contains a slight amount of radioactive substance, and if even one .alpha. (alfa) particle (He.sup.2+) emitted from the radioactive substance enters into the DRAM, then the amount of stored charges, i.e. the stored contents will change, but the information thus changed can be restored by re-writing. Since the DRAM itself is not destroyed, this phenomenon is called a soft error. The presence of such a soft error hinders the miniaturization of the DRAM.
(ii) Latch-up
CMOS devices are now widespread as power-saving devices. The CMOS devices include many parasitic elements, and hence perform an unexpected operation according to the conditions under which they are placed. A phenomenon which is particularly liable to occur is one in which a large current flow is triggered under a certain condition, resulting in the destruction of the device. This phenomenon is called latch-up. When .alpha. (alfa) particles enter the CMOS device, electrons and holes are generated in large quantities, triggering the latch-up. The presence of the latch-up constitutes an obstacle to the miniaturization or microfabrication of the CMOS devices.
A micro ion beam (MIB) and a focused ion beam (FIB) are both focused ion beams although there is a slight difference in nuance between the two terms, and differences between them are given in the following table.
TABLE 1 ______________________________________ Comparison between micro ion beam and focused ion beam MIB FIB ______________________________________ Ion particle light ion heavy ion Acceleration high (.about.MeV) low to medium energy (.about.hundreds of keV) Beam diameter .about.1 .mu.m tens of nm Beam parallelism parallel nonparallel Use analysis fine surface machining ion implantation ______________________________________
FIG. 14 shows the construction of a conventional ion microprobe system, and FIG. 15 is its block diagram. In FIGS. 14 and 15, reference numeral 1 indicates a Y steerer, 2 a feedback slit, 3 a deflector, 4 a beam viewer, 5 a beam shifter, 6 a beam profile monitor, 7 a pre-slit, 8 a micro-slit, 9 a turbo pump, 10 a scraper slit, 11 a Faraday cup, 12 a quadrupole magnet lens, 13 a beam scanner, 14 a SEM column, and 15 a target chamber.
The ion microprobe system shown in FIG. 14 and a beam focusing technique are disclosed in detail in Y. Nakata, M. Sugimori, M. Ishikawa, H. Shimizu and I. Ohdomari, "Development of Ion Microprobe," Bulletin of Science and Engineering Research Laboratory, Waseda University, No. 124, pp. 20-33, 1989 and in T. Kamiyama, E. Minehara, K. Mizuhashi, Y. Nakada and I. Ohdomari, "Doublet Q-Magnet for MeV Ion Microprobe," the same bulletin as mentioned above, pp. 34-38. The beam focusing technique is a technique which focuses high-energy ion beams while retaining their parallelism. The minimum beam spot size now available is 1.9.times.1.7 .mu.m.
The beam focusing technique is related to the irradiation accuracy of the ion irradiation system according to the present invention. Since a single ion that the present invention handles passes the same orbit as that of the ion beam, minimization of the beam spot size is needed to ensure accurate irradiation.
The ion beam is made flat by use of a slit, which is operated in the same manner as that in which the aperture of a lens is stopped down. The aperture of an objective slit is made as small as possible so that it can be regarded as a point light source. (The opening of the objective slit can be adjusted by increments of 1 .mu.m.)
Ion microbeams, which are high-energy charged particle beams having their diameters reduced down to several submicrons, have already been employed as means for an elemental analysis or the like at more than twenty places throughout the world. Much effort has been devoted, in particular, to increasing an ion current in order to obtain better statistics. However, increased ion current density now poses a serious problem because of radiation damage to the sample which is caused by a concentrated application of ions to an extremely limited area of the sample.
To reduce the radiation damage to the minimum, the prior art ion microprobe uses a high-resolution, high-efficiency radiation detector, and the conventional ion irradiation system is provided with a scanning electron microscope (SEM) so that it is capable of irradiation by both of the ion microbeam and an electron beam.
The ion irradiation system has been developed primarily for investigating the effect of irradiation of the sample by high-energy charged particles. A technique for controlling ions to strike the sample at a specified point is now being developed in GSI of Germany as well, but it does not include control of the number of incident ions. Besides, an experiment which uses radioisotopes such as .sup.24.spsp.1 Am is also conducted to examine the effect of irradiation by a small number of ions, but in this case the position of incidence of ions on the sample cannot be decided and ions of arbitrary energy cannot be obtained either.
FIG. 16 through 18 are schematic diagrams for explaining techniques for directing the above-mentioned collimated ion microbeam to the target position or point on the sample surface.
A first step is to detect the positional relationship between a reference position and the target position by means of the scanning electron microscope (SEM) (FIG. 16). The next step is to move the sample holder of table so that the ion beam strikes at the reference position (FIG. 17), followed by moving the sample holder by the distance based on the detected positional relationship to bring the target position into coincidence with the reference position (FIG. 18). Thus, the ion beam can be applied to the sample at the desired strike position.
A fine-structured silicon (Si) test pattern is placed at the reference position. FIG. 19 shows an SEM image of the silicon test pattern.
The pattern is composed of lines and spaces (0.5 to 5.5 .mu.m wide) and is designed to permit measurement of both of the position of incidence of the ion beam and the beam spot size. Such a computer output as shown in FIG. 19 is obtained by scanning the ion beam. In this instance, if two adjacent lines can be distinguished from each other, then the ion beam spot size is estimated to be equal to the space defined by the two lines therebetween.
As mentioned above, the silicon test pattern has two functions, i.e. a standard of the beam spot size and a reference position for aiming the ion beam at a specified point. The above-noted aiming technique is needed because direct irradiation of the sample with the ion beam will cause damage to the sample and hence is not preferable.
A description will be given hereunder of the prior art which are related to the ion irradiation system and method according to the present invention.
Techniques for the detection of the beam spot size and its position of incidence for the ion microprobe system are disclosed in M. Sugimori et al., "Beam size and beam site detecting system for ion micro probe," Proc. of '89 Autumn Meeting of the Applied Physics Society of Japan, 29a-H-2, pp. 494. To facilitate the detection of the spot size and position of incidence of the ion beam, the detecting system set forth in the above prior art literature utilizes a target chamber provided with a scanning electron microscope (SEM) so that a secondary electron image by the ion beam and an SEM image are observed in the same target chamber. This system uses, as the test pattern, a fine-structured relief test pattern formed on a silicon chip and detectes the spot size and position of incidence of the ion beam by comparing the secondary image and SEM image of the silicon test pattern. By rotating the sample holder, the sample is set at 90 degrees to the electron beam during observation of the SEM image and similarly set at 90 degrees to the ion beam during radiation of ions. That is, the rotational angle of the sample holder is variable. With this detecting system, a 1 .mu.m pattern can be observed with high resolution.
The measurement of the beam size is disclosed in T. Kamiya et al., "Beam Size Measurement for MeV Ion Microbeam System," Proc. of '91 Spring Meeting of the Applied Physics Society of Japan, 29p-ZD-11, pp. 547. Kamiya et al. measured the beam size by use of an He+beam of 3 MeV in order to evaluate the performance of a microbeam unit mounted on a 1.7 MV tandem accelerator. Experiments reveal that it is effective in measurement of the beam spot size to employ a method which detects secondary electrons which are generated by scanning with a microbeam a fine-structured silicon relief test pattern fabricated by semiconductor machining techniques. As the result of a preliminary measurement, a beam spot size of 1.7.times.1.9 .mu.m.sup.2 was obtained.
A beam aiming technique is disclosed in M. Koh et al., "Beam site control system for micro ion beam," Proc. of '91 Spring Meeting of the Applied Physics Society of Japan, 29p-ZD-12, pp. 547. The disclosed system employs a target chamber with a scanning electron microscope (SEM) and applies as ion beam and an electron beam to a fine-structured silicon relief test pattern to obtain its secondary electron images, from which the position of irradiation is detected. Positioning control for irradiation is effected by (a) detecting a reference irradiation point, i.e. the position of the sample and then (b) transferring the sample holder and irradiating the sample with the ion beam.
The above-mentioned beam aiming technique has the following advantages:
(1) It has been confirmed that the spot size of and He+beam (3 MeV) can be reduced by the edging effect to 1.7.times.1.9 .mu.m.sup.2.
(2) It is possible to carry out real-time observations of a secondary electron image by the ion beam on the SEM display screen and perform image processing by a computer.
(3) It is possible to observe an ultra-fine relief test pattern (which consists of lines and spaces 0.5-5.5 .mu.m wide) and effect high-speed, simple irradiation position control, using the fine relief test pattern as a reference position.
The concept of the single ion irradiation system is set forth in J. Murayama, et al., "Development of Single Ion Hitting System," 1990 Autumn Meeting of the Applied Physics Society of Japan, Proceedings, 27p-Y-1. The single ion irradiation system disclosed in this literature is formed by an ion microprobe and a beam chopper. The beam chopper includes a slit, a deflector and an electric field control circuit and is used to extract a single ion from an ion microbeam.
The single ion irradiation system operates following the procedure mentioned below.
(1) The ion beam is aligned and the position of irradiation is determined.
(2) A bias voltage is applied to the deflector to deflect the ion beam to prevent it from passage through the slit.
(3) After a certain elapsed time a reverse bias is applied to the deflector to deflect the ion beam in the reverse direction. The thus reverse-deflected beam is also prevented from passing through the slit. (At this instant a single ion is extracted.)
(4) Secondary electrons from the target are detected and the quantity of radiation is identified.
However, the Murayama et al. literature does not confirm the extraction of a single ion nor does it disclose the entire structure of a concrete ion irradiation system and a specific method of extraction of a single ion or predetermined number of ions. Further, the literature makes no mention of specific conditions for extracting a single ion or desired number of ions with a high degree of accuracy.
Moreover, the extraction of a single ion by chopping an ion beam had not been ascertained experimentally. But the inventors of the subject application have now ascertained the extraction of a single ion; namely, they have succeeded in experimentally extracting a single ion and a predetermined number of ions with accuracy. The present invention concerns an ion irradiation system and method for obtaining a single ion and a predetermined number of ions.
Experimental data on the extraction of a single ion is disclosed in the inventors' report, "Singularity Estimation of Ions Extracted by Chopping Ion Beam," Proc. of '91 Autumn Meeting of the Applied Physics of Japan, 12p-ZB-5, pp. 591. Furthermore, the irradiation of CMOS ICs with ions is reported by the inventors' in "Mapping of Radiation Immunity on CMOS Devices," Proc. of the above-noted meeting, 12p-ZB-6.